Geoscience Reference
In-Depth Information
become acidic much more readily than clay soils because they lack this buffering ca-
pacity.
S
OIL PORES, ATMOSPHERE AND WATER
Lastly, in this survey of the soil environment, we return to the question of space within
the mineral soil. Permeating the solid mass of mineral and organic particles there is a
labyrinth of inter-connecting channels and pores differing greatly in size. These can
be divided into capillaries or micropores less than 0.03mm (30 microns) in diameter,
medium-sized mesopores, in the fine sand particle range of 0.03-0. 1mm, and large
macropores of more than 0.1mm or 100 microns. The capillaries serve as sites of
activity for root hairs and micro-organisms - they are the laboratories of soil life. The
larger pores act as service ducts allowing rapid movement of air, water and dissolved
nutrients through the soil. They are also the living quarters for a wide range of true
soil animals. Here, for example, we can find an assortment of protozoa, and a mul-
titude of tiny nematodes (eelworms) which can scarcely be seen by the naked eye,
small mites, white Protura and Thysanura (bristle-tails), blind and springless spring-
tails, and multi-legged centipedes, pauropods and symphylids. These are mainly soft-
bodied creatures, unable to burrow for themselves, but even the burrowing fauna,
such as worms and slugs, millipedes and woodlice, the larvae of beetles and moths,
and sundry others, make use of the larger pores and channels when they can. Soil an-
imals, plant roots, fungi and most bacteria need oxygen which can enter the soil eas-
ily through air-filled pores from the atmosphere above. Oxygen can diffuse through
water-filled capillaries, but 10,000 times more slowly. In soil crumbs that are more
than 3mm in diameter, all the available oxygen may be used up before it can reach the
centre. Such conditions allow anaerobic bacteria to survive in most soils, including
those like some
Clostridium
species that perform the important function of fixing ni-
trogen from soil air. Another group of microorganisms, the actinomycetes, can flour-
ish in intermediate conditions of low oxygen tension. The activities of the micro-flora
are described in
chapters 6
and
9
.
Soil air also contains carbon dioxide given off in respiration and by the decom-
position of cellulose and other plant remains. Although the actual carbon dioxide con-
centration is low compared with that of oxygen or nitrogen, only about 0.5 per cent
by volume, this is already ten to twenty times more than in atmospheric air. Small
fluctuations in carbon dioxide concentrations therefore can have important direct and
indirect effects on plant growth. Microbial activity can also produce ammonia, marsh
gas (methane), hydrogen sulphide and other gases. Our sense of smell is very limited
so we are unable to appreciate the galaxy of aromas that soils must have, but we can
usually tell if a soil is sweet smelling or unpleasant, especially when anaerobic. With-
